1. Field of the Invention
The present invention relates to a method and apparatus for handling MBMS dynamic scheduling information, and more particularly, to a method and apparatus for determining the Modulation and Coding Scheme (MCS) of an MBMS dynamic scheduling information in a network terminal of a wireless communication system.
2. Description of the Prior Art
To enhance multimedia performance of the 3G mobile telecommunications system, the 3rd Generation Partnership Project (3GPP) introduces a Multimedia Broadcast Multicast Service (MBMS), which is a point-to-multipoint bearer service established on an existing network architecture of the Universal Mobile Telecommunications System (UMTS). MBMS allows a single source terminal to simultaneously transmit data to multiple user equipments (UEs) via Internet Protocol (IP) packets.
However, as the multimedia performance of mobile devices advances, consumers are more interested to have multimedia or mobile TV services via the mobile devices. In order to meet such requirement, the 3GPP introduces an enhanced MBMS (eMBMS) in a specification of long term evolution (LTE) Release-9, to support high quality streaming multimedia and real-time MBMS services.
The eMBMS introduces a single frequency network (SFN) operation for MBMS transmission, i.e. MBMS Single Frequency Network (MBSFN), to reduce service interruption due to frequency switching during transmissions. In MBSFN, single frequency is used by multiple cells to perform synchronized transmission at the same time, so as to save frequency resources and enhance spectrum utilization.
In addition, only two logical channels are defined in eMBMS to support point-to-multipoint (p-t-m) downlink transmission: Multicast Control Channel (MCCH) and Multicast Traffic Channel (MTCH). MCCH is utilized for transmitting control messages of all MBMS services in an MBSFN, and MTCH is utilized for transmitting session data of an MBMS service. Both MCCH and MTCH are mapped to a transmission channel newly defined by eMBMS, i.e. Multicast Channel (MCH).
For the current specifications, 3GPP change request R2-093533 is a stage 2 update for the MBMS functionalities. The following description about multi-cell transmission is quoted from this document:
(1) Synchronous transmission of MBMS is performed within an MBSFN Area.
(2) Combining of MBMS transmission from multiple cells is supported.
(3) A single transmission is used for MCH (i.e. neither blind HARQ repetitions nor RLC quick repeat);
(4) A single Transport Block is used per TTI for MCH transmission, and the TB uses all the MBSFN resources in the sub-frame.
(5) MTCH and MCCH can be multiplexed on the same MCH and are mapped on MCH for p-t-m transmission.
In general, an MBSFN has an MCCH. However, when an enhanced Node B (eNB) is simultaneously covered by multiple MBSFN areas, the eNB may have multiple MCCHs. Besides, since an MBSFN area can simultaneously support multiple MBMS services, and different MBMS services may have different requirements, such as Quality of Service (QoS), Block Error Rate (BLER), according to different characteristics, an MBSFN area may have multiple MCHs. Different MCHs suffice requirements of different MBMS services by applying different modulation and encoding schemes. MCCH is mainly responsible for providing these MCHs with the following control parameters:
(1) Modulation and Coding Scheme (MCS): illustrating encoding and modulation scheme of each MCH.
(2) MBMS service list: listing MBMS services currently provided by each MCH, and including MBMS service ID and session ID, and further including corresponding Logical channel ID (LCID) of MBMS services. Noticeably, MBMS service list only provides session information of MBMS services currently processed. In other words, once a session of one of the MBMS services is ended, a network terminal removes session information of the MBMS service from the MBMS service list, to notify a UE related to the MBMS service that the MBMS service session has been ended.
(3) MCH Sub-frame Allocation Pattern (MSAP): defining a pattern of sub-frames used by each MCH in a scheduling period. A set of these sub-frames used by one MCH is called an MSAP occasion. In each MSAP occasion, a network terminal can multiplex MCCH and MTCHs corresponding to different MBMS services on a Physical Multicast Channel (PMCH), which is a physical layer channel of MCH, for transmission.
Since an MCH may simultaneously provide multiple MBMS services, while an MBMS UE may be only interested in one of the services, the current specification further provides a Dynamic Scheduling Information (DSI) for an MCH, to indicate the sub-frames used by each MTCH in an MSAP occasion. The following principles are used for the Dynamic scheduling information:
(1) The dynamic scheduling information is used without considering whether multiple services are multiplexed onto an MCH to transmit or not.
(2) The dynamic scheduling information is generated by the eNB, and is transmitted at the beginning of each MSAP occasion, i.e. a first sub-frame.
(3) The dynamic scheduling information allows a UE to determine which sub-frames are used by each MTCH.
(4) How a dynamic scheduling information is carried is for further study, e.g. the dynamic scheduling information can be carried in a MAC Control Element or transmitted via a separate logical channel, such as a Multicast Scheduling Channel (MSCH).
(5) A mapping relation between an MTCH and corresponding sub-frames is determined according to indexes of sub-frames belonging to each MSAP occasion.
Under such a situation, 3GPP change request R2-093093 proposes to use an MBMS scheduling info MAC control element for carrying the dynamic scheduling information. In an MBMS scheduling info MAC control element, each MBMS service contains one ending point index (EPI) field for each MBMS service, for indicating an index of an ending sub-frame for each MBMS service in the current scheduling period.
In addition, except that different MCHs may apply different MCSs, 3GPP also proposed to have a logical channel specific MCS to support different QoS for each logical channel transmitted on the same MCH. For example, different MTCHs transmitted on the same MCH may have different MCSs, or MTCHs and MCCH transmitted on the same MCH may have different MCSs.
From the above, it is clear that the dynamic scheduling information may be transmitted together with the MCCH or an MTCH on an MCH/PMCH in the same sub-frame. Since the MCCH carries the MBMS service list and reception related information, e.g. the MTCH/PMCH configuration associated with each service, it is believed that the error rate requirement of MCCH is more critical than that of the dynamic scheduling information because a UE cannot perform reception of any MBMS service before successful reception of the MCCH. Thus, it is straightforward to apply the MCS of the MCCH for encoding or decoding the PMCH when the dynamic scheduling information is transmitted together with MCCH in the same sub-frame.
However, in the current specification, the MCCH is transmitted every MCCH repetition period, while the dynamic scheduling information is transmitted at the beginning of the MSAP occasion. Because the MCCH repetition period may be different from the period of an MSAP occasion, the dynamic scheduling information may be transmitted in a sub-frame without being accompanied by the MCCH. In this case, the dynamic scheduling information may be transmitted in a sub-frame together with the MTCH. AT the first glance, it seems the MCS of the present MTCH can be applied for transmitting the dynamic scheduling information in this case. But, before the dynamic scheduling information is received, a UE cannot know which MTCH is transmitted with the dynamic scheduling information. So it is infeasible.
Therefore, it is still not clear how to determine the MCS for receiving the dynamic scheduling information when the MCCH is not present in the same sub-frame. As a result, the dynamic scheduling information may not be received by the UE correctly.